The present invention is directed to an optical star coupler for multi-mode fiber optical waveguides in which two bundles of optical fibers are connected to one another by a mixing element consisting of a planar waveguide having a thickness which is approximately equal to the diameter of the fibers and the incoming and outgoing group of fibers lie in the same plane as the planar waveguide so that the packing density is as high as possible.
It has been proposed in U.S. patent application Ser. No. 136,423, filed Apr. 2, 1980, which issued as U.S. Pat. No. 4,362,357 on Dec. 7, 1982 and was based on German Pat. No. 29 15 114.2, to provide an optical star coupler for multimode light conducting fibers which coupler has a mixing element formed by a planar waveguide having a thickness approximately equal to the diameters of the fibers which are connected to the input and output ends of the mixing element and the fibers of the incoming group and outgoing group as well as the mixing element are all positioned substantially in the same plane.
In comparison to three dimensional embodiments of the coupler, the optical star coupler of the above-mentioned application exhibits low insertion losses. Therefore they are particularly suited for use in optical communication technology particularly in data buses in which high path attenuation occurs because of the large distances between the subscribers and/or the large number of subscribers involved in the information exchange. The planar waveguide, as a mixing element, guarantees a good mixing of all modes and thus provides as uniform as possible a light distribution to all fixed output fibers in comparison to all input fibers. Each of the input and the output fibers of the optical star coupler are connected to a system fiber for example in a data bus.
The thickness of the planar waveguide was made equal to the diameter of the fiber optical waveguides of the input and the output groups so that the coupling losses on transition from the fiber optical waveguides to the mixing element and vice versa are as low as possible. Moreover, the jacket thickness of the fiber optical waveguides are kept as thin as possible.